All laboratory light sources are, to some extent, non-uniform across their beam front, both in time and in space. Therefore, a light source imaged upon a screen exhibits both hot and cold spots which fade and brighten, semi-uniform gradiations which fluctuate with time, and a general waxing and waning of overall intensity. In an incandescent lamp, the variations are due to asperities of the glass envelope, the shape and position of the filament, and time changes in the filament material such as oxidation or crystal growth. A laser suffers from defects in its optics, contaminants on its optical surfaces, and lasing fluctuations. While the degree of these problems varies over orders of magnitude, they are always present to some extent and must be considered if one is attempting to use a light source at its highest resolution.
If a light beam is used in such a way that a normalizing (i.e., reference) signal is obtained from one part of the beam front, and a measurement signal is obtained from another part, then there will be inherent errors in the normalized output due to the time and spatial variations alluded to above.
There are three ways around this problem. The first solution is to somehow homogenize the beam in time and space. The second solution is to obtain both the normalizing signal and the measurement signal from the same portion of the beam, and to alternate samplings at intervals much shorter than the time variations of the light source. The third solution is of course to just determine that the errors are below the required system accuracy.
Several U.S. patents exist in the field of utilizing radiant energy to inspect the diameter of a moving cylindrical object. One of these patents is U.S. Pat. No. 3,604,940 issued to David R. Matthews. As shown in FIG. 1, two highly collimated beams of light are directed tangentially along diametrically opposite surface portions of the object so as to be partially intercepted thereby. These partially intercepted beams are focused on two detectors and then compared to determine the size of the sample. Therefore, since two light beams are used, each signal would fluctuate differently with respect to time and space and errors in the readings would result. In addition, even though a beam splitter is used in FIG. 6 to determine the diameter of very small objects, two different light beams are still employed and, similarly, errors would result.
U.S. Pat. No. 3,856,412 issued to Carl A. Zanoni describes an electro-optical sensor for providing an output proportional to the cross-section of an object, such as the diameter of a cylinder which utilizes only a single beam of laser light. The diameter is determined by photoelectrically sensing when the laser beam is modulated on or off by the edge of the object, using the first and second derivatives of the output of the sensor. An auxiliary photoelectric output is obtained by passing a part of the scanning laser beam onto a sensor over a precision grading or scale with markings. This operation generates a modulated signal whose spatial frequency is independent of variations in the speed of scanning and intensity of the laser beam. Therefore, it can be seen that the Zanoni patent calculates the diameter of the signal using a time duration of the signal rather than the signal's intensity.
Therefore, a review of the prior art has failed to discover any patent or other reference which determines the diameter of a moving cylindrical object utilizing only a single beam of laser light with the measurement being based upon the shadow diameter of the sample.